The snapback characteristics of different NPN bipolar junction transistors (BJTs) and Bipolar SCRs (BSCRs) are particularly important when devices are used both as self-protecting as well as stand-alone devices.
While the collector in such devices can typically be large, emitter length is limited. Due to self-alignment considerations in bipolar technology, only a few emitter dimensions are supported. In the case of some technologies such as Si—Ge, only a single emitter length is, in fact, supported. The base contact region, which forms part of the self-aligned bipolar design, also faces size constraints. A typical BIT as known in the art is shown in FIG. 1, which shows an n-emitter 100 formed in a p-base 102, which is, in turn, formed in a n-type collector 104. The emitter 100 is contacted through an emitter poly layer 110, while the base 102 is contacted directly by a base contact 112, which contacts a silicide layer (not shown) on the base 102. The collector 104 under the base 102 is contacted via a buried layer 106 and a Sinker 108 as known in the art. The sinker 108 is contacted by a sinker contact 116, which is spaced from the emitter poly 110 by an oxide layer 118. Thus the vertical configuration of the BIT is achieved by providing the sinker 108 that extends vertically, and the horizontally extending buried layer 106.
The size limitations of the emitter 100 limit its current carrying capacity and therefore create problems in using the device in high current ESD applications. In practice this issue is addressed by making use of multi-emitter BIT and BSCR devices to support the high ESD currents. For purposes of this application the term multi-emitter covers individual emitters that are connected together as well as an emitter with multiple emitter fingers. The individual elements of the emitter, whether individual emitters or emitter fingers of a single emitter, will be referred to as emitter fingers for convenience. Different snapback bipolar and BSCR configurations have therefore been implemented by placing multiple bases and emitters between collectors, e.g., C-B-E, C-B-E-B-C, C-B-E-B-E-B-C, C-B-E-B-B-E-B-E-B-C, etc., or around a collector, e.g., B-E-C-E-B. One such configuration is shown in FIG. 2, which shows a cross section through a multi-finger emitter BJT with emitter fingers 200, 202, 204, 206, 208, and multiple base contacts 210, 212, 214, 216. In FIG. 2, only one collector contact 220 is shown on the right hand side, but the multiple emitter fingers and base contacts could also be formed between a pair of collector contacts with another collector contact formed on the left hand side.
While multiple emitter fingers and multiple base contacts seek to address the current density concerns during ESD events, test results indicate that the use of multiple emitters or multiple bases results in non-uniform current distribution at high current densities. As a result, for ESD operation, which involves high current densities, the devices will not perform optimally. Typically proper current distribution is only observed under normal operating conditions as opposed to the operation during ESD events. This can be ascribed to the fact that the overall ESD operation depends on the sub-collector design, which is typically designed to provide proper current distribution during normal operation as opposed to during ESD operation.